Many different orthopedic casting materials have been developed for use in the immobilization of broken or otherwise injured body limbs. Some of the first casting materials developed for this purpose involved the use of plaster of Paris bandages consisting of mesh fabric (e.g., cotton gauze) with plaster (e.g., calcium sulfate hemihydrate) incorporated into the openings and onto the surface of the mesh fabric. Plaster of Paris casts, however, have a number of attendant disadvantages, including a low strength-to-weight ratio, resulting in a finished cast that is very heavy and bulky. In addition, plaster of Paris casts develop their strength over a relatively long period of time. This makes it necessary to avoid weight bearing situations for up to 48 hours. Furthermore, plaster of Paris casts typically disintegrate in water. This makes it necessary to avoid bathing, showering, or other activities involving contact with water.
Polycarboxylate cement casting materials that contain finely divided calcium aluminosilicate glass and calcium fluoroaluminosilicate glass are known. The calcium aluminosilicate or calcium fluoroaluminosilicate glasses are combined with a polycarboxylic acid, which when mixed with water set to form a solid mass. Such glass-containing polycarboxylate cements are generally expensive to make and often have slow set-up times (e.g., greater than about 10 minutes). Additionally, fluoroaluminosilicate glasses are undesirable because they are very expensive to manufacture due to the presence of fluorine, which requires strict environmental controls.
More recently, orthopedic casting tapes have been produced using curable resins coated or impregnated onto a substrate. Typically, the casting tape is stored as a roll in a water-impermeable storage pouch until needed for use. When needed, the roll of tape is removed from the pouch and contacted with water. Generally, the casting tape includes a water-curable resin Thus, shortly after the tape is dipped in water, the resin will begin to cure and the tape harden. The medical practitioner applies the casting tape to a patient immediately after it has been soaked with water. Generally, resin materials are chosen that begin to cure immediately after being dipped in water, and that will set (i.e., harden) sufficiently to resist passive motion in ajoint (e.g., a wrist or ankle) in about 3 minutes to about 5 minutes, and to be weight bearing within about 30 minutes.
Typical resin coatings utilized in conventional orthopedic casting systems include isocyanate-functional urethane prepolymers. Other known systems include nonisocyanate alkoxysilane-terminated urea and/or urethane prepolymers. Generally, such resin coatings also include catalysts to facilitate reaction with water. Lubricants may also be used, e.g., to facilitate unrolling, application, and molding. Furthermore, defoaming agents may be used to maintain porosity while carbon dioxide is evolved during the curing process.
Typical substrates upon which the above resin coatings are applied to produce an orthopedic casting tape are fabrics prepared from glass and/or synthetic fibers. Fiberglass or other high modulus fibers contribute significant strength to the cured resin substrate composite as well as provide a reservoir for the resin during storage and end-use application of the casting tape. Nonglass low modulus substrates generally provide for a lower weight and more radiolucent cast. With most nonglass substrates the strength is limited by the amount of resin that can be held by the substrate. High resin loading with materials known to date must generally be avoided, however, to prevent excessive heat build-up in the cast while it is in its early stages of cure, i.e., during hardening, as a result of the exotherm produced in the above resins.
A need still exists for a hardenable composition that can be used in a wide variety of orthopedic support materials, whether used in combination with a fiberglass substrate or a nonglass low modulus substrate, particularly one that is more economical and/or environmentally friendly than conventional compositions and can withstand exposure to water once hardened.